Back in business: Sadiq Khan on the South Bank the day after winning Labour’s candidacy for London mayor Daniel Hambury

Less than 24 hours after his surprise victory in Labour’s mayoral contest Sadiq Khan is back in business. When we meet on the South Bank in the grey light of Saturday morning for his first newspaper interview since he was selected he has had just three hours sleep, after touring the broadcast studios and celebration parties the night before.

Yet, buoyed by his win, he is full of energy, teasing the photographer that his pictures should portray him as the best-looking candidate in the mayoral race.

There are no doubts where his sights are now aimed: Zac Goldsmith, handsome Tory MP, millionaire environmentalist, and favourite to win the Conservative candidacy.

When I mention Goldsmith, Khan launches into what is clearly going to be one of Labour’s central lines of attack against the Tory hopeful.

‘I bring real-life experience to this. I know what it’s like worrying if you have enough money to pay bills’

Sadiq Khan

“One of the questions people need to ask before next May is can they afford to take a risk casting their vote for a mayor whose previous views are very anti-European?” he said.

“The EU is a huge market and we’d be taking a big risk with London’s future electing a mayor who was not passionately pro-European.”

For good measure he lists his own pro-EU credentials, although his new leader’s are lukewarm.

Khan is more cautious when it came to Goldsmith’s gilded up-bringing. “Zac’s background doesn’t exclude him from having empathy,” he said.

However, he added: “I bring real-life experience to this. I know what it’s like worrying if you have enough money to pay the bills at the end of the month. I know what it’s like not having a secure, affordable home.”

He had harsh words for Boris Johnson, claiming he had given the impression mayors could not achieve much: “That’s just not true. I’ve met and seen what mayors around the world can do if they roll up their sleeves and get going.”

Later that morning Khan, 44, was at Westminster to hear Jeremy Corbyn confirmed as Labour Party leader. I watched as Corbyn told him “Sadiq, we’re going to be campaigning together”. Khan’s face was inscrutable.

One senior Tory told me months ago Labour would “cut Sadiq off at the knees” if he won the nomination, though Goldsmith is understood to view him as a tough opponent.

There are already suggestions that Lynton Crosby, who is working with Goldsmith, has been thinking up slogans linking Khan to his new boss.

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But Khan is undaunted at being the target of Tory fire, describing the formidable Australian elections strategist as “a lovely man” who will run a “nice, positive campaign”. “Look, I’m not scared by my opponent or the campaigns they’re going to run, I’m excited about being Labour’s candidate and my campaign,” he said.

He will have to win over Tory-leaning outer London — taking on Crosby’s “doughnut strategy” — but says he will have potential supporters who have been priced out of central London.

One of Khan’s greatest strengths is his back story, and he talks often about growing up with seven siblings on a council estate in Tooting, and how he was inspired by his bus driver father.

He was still sleeping in a bunk-bed at home at 24, while he was a trainee lawyer, to save a deposit for a house.

He still lives in Tooting with his lawyer wife Saadiya and their two teenage daughters and attends the local mosque. He says he is “very proud” of being Muslim but that like most Londoners, he has multiple identities. While Khan is charming and persuasive, he has a steely glint in his eye and that politician’s swift patter that makes many of his lines sound rehearsed.

He is a formidable tactician: he knows all the tricks of the trade and is quite prepared to be ruthless.

During the selection campaign his opponents accused him of “playing politics” over Heathrow expansion — he switched from supporting a third runway to opposing it. He backs expansion at Gatwick instead.

“I don’t apologise at all,” he said. “Indeed I want to make a virtue of the fact that when the facts [on air quality] changed, I changed my mind.”

When he was first elected an MP in 2005 he voted against 90-day detention without charge. Yet three years later he was a whip pushing through the 42-day version.

He is undoubtedly a pragmatist. “We’ve got to have the humility to recognise that the British public didn’t vote for us in 2015 and you’ve got to learn lessons,” he said. “Our ability to improve the quality of life of Londoners is only fulfilled if we win elections.”

But he is also a fighter. As a child he learnt to box to look after himself on a tough estate. “There arguably hasn’t been a tougher time to be Mayor than the next four years. That’s why London needs a fighter,” he said.

“There will be times I’ll be arguing with Government, but there will be times I need them on my side.”

He will also have to persuade a section of the electorate that Labour is not anti-business. He said: “I want to be the most pro-business Mayor ever. I’m going to work with business to make sure we get the best deal.”

Khan does not seem overwhelmed at the scale of the challenge ahead.

Ken Livingstone, who endorsed his candidacy, is on hand to share experience and advice. Would he bring Livingstone back to City Hall if he wins? “I take counsel from whoever provides it,” he said. “I want to be going to people and listening for their ideas. I’m happy to listen, within reason, to anyone.”